This project is concerned with certain features of the maternally inherited condition referred to as "sex-ratio", SR, in species of the neotropical willistoni group of Drosophila. SR has proved to be a consequence of trans-ovarial transmission of micro-organisms, now demonstrated to be related to spiroplasmas, from generation to generation and is typically expressed as the absence of males in the adult progeny of SR females. Absence of males is a consequence of early zygote mortality through action of an agent associated with, or produced by, the SR-spiroplasmas. The spiroplasmas are lysogenic with associated phages. These features are being investigated in an attempt to assess the relative roles of phage and spiroplasmas, as well as the basis for the susceptibility of male and the resistance of female zygotes to the action of the killing agent. SR strains in four different species distributed over northern South American and the Antilles are included in these studies.